It is a well known fact that the aerodynamic performance of the external shape of a vehicle and its movement through a medium can be defined in a certain non-dimensional quantity called the drag coefficient CD. The drag coefficient at lower speeds of a body is mainly dependent on the aerodynamic configuration of the body and the Reynolds number, which is a measure for the ratio of inertia forces to viscous forces in a flow. The drag coefficient and the corresponding aerodynamic forces are directly related to the driving speed to the second power, and to the fuel consumption, thus the operational economics, of that corresponding vehicle.
Heavy road transport vehicles can be characterized in an aerodynamically sense as bluff shapes. This means that the aerodynamic properties of these road vehicles are strongly influenced by flow separation. Flow separation occurs when the boundary layer, which is a thin layer that bridges the velocity difference between the moving vehicle and the lower air speed, encounters a sufficiently large adverse pressure gradient due to, for instance, abrupt geometrical changes in the body like for instance at the back of bluff road vehicles.
The term bluff most commonly refers to bodies which have leading-edge flow separation, as most vehicles do at large side wind angles. The flow which is touching the front of the vehicle, goes, for instance, along the side of the trailer to the back of the same trailer where it is not able to follow a 270 degrees corner comprising the side and back surfaces. For example the squared edges found at most corners of bulk commercial road and rail cargo carriers. The effects of these flow separations are most apparent in their high aerodynamic drag levels, where the pressure drag component is many times higher due to flow separation than the drag due to skin friction as with airfoils. The aerodynamic drag of a bluff shape is mainly due to the pressure difference of the front and rear faces of the body, with respect to the environment pressure, with only a secondary contribution due to skin friction.
Fuel economy and the associated fuel cost of heavy transport vehicles are very important issues within the operational cost of national and international transport companies. Till this day transportation of goods over the roads is one of the most efficient and flexible methods within the field of freight transportation. A large amount of engine power is required to overcome the aerodynamic forces that are acting on a road vehicle, due to the passage of the vehicle through air. Besides reducing the aerodynamic drag with special designed devices or aerodynamically well streamlined bodies, also other measures may positively effect the fuel consumption of vehicles. For instance by introducing vehicle weight reduction through designing lightweight structures, by improving tire friction coefficients which reduce the tire friction forces and by increasing mechanical efficiency of the mechanical parts like the engine, the gear box and the driving shafts. An improved aerodynamic behaviour of a vehicle will, besides an increased fuel economy, decrease the belch of environmental unfriendly exhaust gasses as well as introducing a more save traffic situation because of the decreased tire wear.
Due to the aerodynamic instabilities in the flow around and in the wake behind the vehicle, the vehicle is bucketing slowly over the road which results in tire wear and possible tire burst and thus an unsafe traffic situation.
As a road vehicle is progressing along its path, the volume of air in the near front of the vehicle actually acts as a frontal barrier which causes stagnation drag and thus a loss in fuel economy. Significant advances have already been made in aerodynamic design of the cabin of the tractor and of trucks in general. Rounded cabin corners, side and roof deflectors, aerodynamic mirrors, and side fenders to close the gab between the cabin and the trailer are commonly adopted. Also several aerodynamic devices for the back end of the vehicle, like the boat tail, splitter plates, guiding vanes, air deflectors, and pneumatic systems, are developed and reduce the total drag of the road vehicle significantly. Since the undercarriage of a trailer usually includes transverse chassis beams, a pallet box, axles, support legs, equipment storage volumes, and other irregular elements, this region is characterized by highly turbulent and separated flows.
The present invention relates to vehicles having highly turbulent regions as a result of an interrupted flow at a position of a pivotal connection between at least two parts of the total vehicle combination. Such turbulent regions for example occur typically at the pivotal connection point between a trailer and a tractor or at the position of the connection by a drawbar of one or multiple lorries to a rigid truck. Another example of a vehicle combination is a railway train comprising a locomotive and several wagons. The locomotive and wagons are all pivotally connected to each other. At each connection, regions occur with large pressure differences. At those positions, the flow along the vehicle is interrupted, which adversely affect the aerodynamic behaviour of the total vehicle combination.
U.S. Pat. No. 6,974,178 to Ortega and Salari illustrates several baffle assemblies adapted to be positioned upstream of the wheel assembly for deflecting airflow away from the wheel assembly so as to reduce the incident pressure on the wheel assembly.
A first embodiment of the apparatus of U.S. Pat. No. 6,974,178 shows a wedge-shape side skirt arrangement. The skirt arrangement is mounted on the underside of the vehicle body portion in front of the rear wheel assembly using fasteners or other mounting hardware of a type known in the relevant arts. The skirt arrangement has right and left panels extending down from the underside of the body portion and angled to deflect airflow away from the rear wheel assembly. It is appreciated, that the left and right panels are part of unitary construction and the leading ends thereof may be integrally connected, either at an angle, or with a curvilinear or otherwise continuous shape. The straight panels themselves may also have a concave or convex curvilinear configuration.
A second embodiment of U.S. Pat. No. 6,974,178 shows a wedge-shaped skirt portion with a left and a right panel similar but shorter than the first embodiment and a third forward panel connected to the wedge-shaped portion at a forward location thereof. This third forward panel is centrally aligned with the longitudinal central axis of the trailer.
A third embodiment is compromising a pair of side-skirts which are mounted parallel at or near the transversely opposite side of the body vehicle. In particular, the side skirts may be directly mounted to the underside of the body portion to extend there below, or mounted to the side of the body portion to extend down to a level below the body portion. The side skirts are located near the left and right side lower edges to impede airflow into and across the underside of the trailer.
A first problem of the first and second embodiment of U.S. Pat. No. 6,974,178 is that units, like the battery box, pallet box, storage volume and other necessary parts, which are present on a regular trailer, can not be mounted anymore due to the present side skirts.
Another problem of the three embodiments of U.S. Pat. No. 6,974,178 is the fact that the underside of the body portion of the vehicle is not accessible if necessary for certain tasks like maintenance or storekeeping of parts and the like.
A further problem of the three embodiments of U.S. Pat. No. 6,974,178 is that there are still zones with a high level of turbulence caused by released flows and felt by the passing flow, which adversely affect the aerodynamic behaviour of the vehicle. Especially when the vehicle is subjected to horizontally inclined flows, eddies and heavy irregularities in the flow are occurring.